The OFDM technique distributes data over a number of spectrally overlapping and coherently orthogonal sub-carriers. Although high spectral compactness can be achieved by applying a large number of multiplexed sub-carriers or channels, pulse shaping is also important in OFDM systems. If an OFDM sub-carrier pulse is not properly shaped, sub-carriers located near band edges often interfere their adjacent channels.
An OFDM sub-carrier pulse used for transmission is often chosen to be rectangular. Forming, modulation, and demodulation of rectangular pulses can be implemented by simple and efficient techniques because the rectangular pulse shape generally leads to a sinc function (i.e., sin(x)/x) type of spectrum of the sub-carriers. However, the rectangular pulse of the OFDM sub-carrier often has relatively large power spectral side lobes that fall off as f−2. Thus, spectral efficiency in the rectangular pulse OFDM system often is not optimized.
Certain techniques have been developed to shape the OFDM pulse to suppress side lobes. For example, U.S. Pat. No. 6,999,503 issued Feb. 14, 2006, to Vadde discloses a cyclic convolver deployed in the frequency domain which may suppress plurality of sub-symbols in the time domain to enhance the bit-rate by dropping a portion of the time domain signals. However, such conventional correlative coding techniques may introduce inter-symbol interference (ISI) and may require a separate traditional post-equalizer.
Further, in wireless communication, wireless channels often have spectral nulls occupying successive sub-carriers. Errors may be bursted if there is an error in one sub-carrier. Thus, OFDM systems with such correlative coding techniques often have relatively high bit-error rate (BER). Moreover, when a cyclic prefix is inserted in a correlatively coded OFDM signal, the waveform of the signal may be non-continuous and the OFDM system may only achieve relatively low spectral compactness.
Methods and systems consistent with certain features of the disclosed embodiments are directed to solving one or more of the problems set forth above.